Thomas A. Minckley

Multi‐decadal drought and amplified moisture variability drove rapid forest community change in a humid region

Climate variability, particularly the frequency of extreme events, is likely to increase in the coming decades, with poorly understood consequences for terrestrial ecosystems. Hydroclimatic variations of the Medieval Climate Anomaly (MCA) provide a setting for studying ecological responses to recent climate variability at magnitudes and timescales comparable to expectations of coming centuries. We examined forest response to the MCA in the humid western Great Lakes region of North America, using proxy records of vegetation, fire, and hydroclimate. Multi-decadal moisture variability during the MCA was associated with a widespread, episodic decline in Fagus grandifolia (beech) populations. Spatial patterns of drought and forest changes were coherent, with beech declining only in areas where proxy-climate records indicate that severe MCA droughts occurred. The occurrence of widespread, drought-induced ecological changes in the Great Lakes region indicates that ecosystems in humid regions are vulnerable to rapid changes in drought magnitude and frequency.

Resilience and regime change in a southern Rocky Mountain ecosystem during the past 17 000 years

Paleoecological records indicate that subalpine forests in western North America have been resilient in response to multiple influences, including severe droughts, insect outbreaks, and widely varying fire regimes, over many millennia. One hypothesis for explaining this ecosystem resilience centers on the disruption of forest dynamics by frequent disturbance and climatic variability, and the resulting development of non-steady-state regimes dominated by early-successional conifers with broad climatic tolerances, such as lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.). To evaluate this hypothesis, we independently reconstructed the vegetation, fire, and effective-moisture histories of a small, forested watershed at 2890 m elevation in southeastern Wyoming, USA, using sedimentary pollen and charcoal counts in conjunction with sedimentary lake-level indicators. The data indicate that prominent vegetation shifts (from sagebrush steppe to spruce–fir parkland at ca. 10.7 ka and spruce–fir parkla...

Decomposing the mid-Holocene Tsuga decline in eastern North America.

The mid-Holocene decline of Tsuga canadensis (hereafter Tsuga) populations across eastern North America is widely perceived as a synchronous event, driven by pests/pathogens, rapid climate change, or both. Pattern identification and causal attribution are hampered by low stratigraphic density of pollen-sampling and radiometric dates at most sites, and by absence of highly resolved, paired pollen and paleoclimate records from single sediment cores, where chronological order of climatic and vegetational changes can be assessed. We present an intensely sampled (contiguous 1-cm intervals) record of pollen and water table depth (inferred from testate amoebae) from a single core spanning the Tsuga decline at Irwin Smith Bog in Lower Michigan, with high-precision chronology. We also present an intensively sampled pollen record from Tower Lake in Upper Michigan. Both sites show high-magnitude fluctuations in Tsuga pollen percentages during the pre-decline maximum. The terminal decline is dated at both sites ca. 5000 cal yr BP, some 400 years later than estimates from other sites and data compilations. The terminal Tsuga decline was evidently heterochronous across its range. A transient decline ca. 5350 cal yr BP at both sites may correspond to the terminal decline at other sites in eastern North America. At Irwin Smith Bog, the terminal Tsuga decline preceded an abrupt and persistent decline in water table depths by approximately 200 years, suggesting the decline was not directly driven by abrupt climate change. The Tsuga decline may best be viewed as comprising at least three phases: a long-duration pre-decline maximum with high-magnitude and high-frequency fluctuations, followed by a terminal decline at individual sites, followed in turn by two millennia of persistently low Tsuga populations. These phases may not be causally linked, and may represent dynamics taking place at multiple temporal and spatial scales. Further progress toward understanding the phenomenon requires an expanded network of high-resolution pollen and paleoclimate chronologies.

Climatic Shifts in the Availability of Contested Waters: A Long-Term Perspective from the Headwaters of the North Platte River

Early summer snowmelt from mountains in northern Colorado and southeastern Wyoming supplies the North Platte River, supporting nationally important agriculture, energy production, and urban development. Repeated decisions from the U.S. Supreme Court have fully apportioned Platte River waters among Colorado, Wyoming, and Nebraska, underscoring societal strains on this system. Now, climate change threatens the regional allocation of water. Tree-ring records indicate that past centuries contained multidecadal “megadroughts” far more severe than those of the historic period. However, the potential for even more persistent droughts, as the result of climate change, is poorly known. We document and evaluate the severity of recent and prehistoric droughts via a combination of data sources: modern temperature, precipitation, and stream gauge data; evidence of low lake-level stands; and related estimates of past hydroclimate change. Modern climate and stream data show an increase in spring temperatures of 2.21°C since 1916, an increase in the frequency of peak spring runoff before 1 May, and a reduction in winter precipitation. Lakes, however, that have only experienced minor hydrologic changes historically were desiccated during prehistoric dry periods during the past 12,000 years. Prehistoric lake shorelines indicate that water supplies were substantially reduced over centuries and millennia, such as from > 8,000 to < 5,000 years before present. The magnitude of these droughts likely also resulted in ephemeral river flows and thus indicates the potential for persistent shifts in regional hydrology. Such shifts should, therefore, be considered as part of long-term economic and legal planning.

A 15,000 year record of vegetation and climate change from a treeline lake in the Rocky Mountains, Wyoming, USA:

Future climate projections predict warming at high elevations that will impact treeline species, but complex topographic relief in mountains complicates ecologic response, and we have a limited number of long-term studies examining vegetation change related to climate. In this study, pollen and conifer stomata were analyzed from a 2.3 m sediment core extending to 15,330 cal. yr BP recovered from a treeline lake in the Rocky Mountains of Wyoming. Both pollen and stomata record a sequence of vegetation and climate change similar in most respects to other regional studies, with sagebrush steppe and lowered treeline during the Late Pleistocene, rapid upward movement of treeline beginning about 11,500 cal. yr BP, treeline above modern between ~9000 and 6000 cal. yr BP, and then moving downslope ~5000 cal. yr BP, reaching modern limits by ~3000 cal. yr BP. Between 6000 and 5000 cal. yr BP sediments become increasingly organic and sedimentation rates increase. We interpret this as evidence for lower lake levels during an extended dry period with warmer summer temperatures and treeline advance. The complex topography of the Rocky Mountains makes it challenging to identify regional patterns associated with short term climatic variability, but our results contribute to gaining a better understanding of past ecologic responses at high elevation sites.

Paleolithic Landscapes and Seascapes of the West Coast of Portugal

The antiquity of coastal adaptations has gained renewed attention in the last several years as archaeologists have recognized that coasts have long been important foci of human settlement (Bailey 2004; Bailey and Milner 2003; Erlandson and Fitzpatrick 2006; Fa 2008; Price 1995; Sauer 1962; Westley and Dix 2006).

The impact of Mt Mazama tephra deposition on forest vegetation in the Central Cascades, Oregon, USA

The eruption of Mt Mazama, c. 7630 yr BP, was the largest North American volcanic event during the Holocene. High-resolution pollen and charcoal analyses were used to examine the impact of Mt Mazama tephra on forest vegetation and possible synergistic interactions with fire activity in the Central Oregon Cascade Range. We selected four small watersheds on a longitudinal transect north of Mt Mazama and recovered lake sediment that spanned the period of tephra deposition. Our sediment records had between 14 and 50 cm of tephra deposited, and we analyzed the sediment at centimeter resolution before and after the deposition horizon in each sediment record. Our analysis shows that nonarboreal pollen percentages and accumulation rates were depressed after Mazama tephra deposition. Recovery to pre-tephra deposition rates occurred after approximately 50–100 years. Arboreal pollen percentage and accumulation rates were less severely impacted, suggesting that the Mazama tephra deposition disrupted understory communities more significantly than overstory species, and that forest communities returned to their pre-tephra-deposition conditions after approximately 50–100 years. Fire events in conjunction with the Mazama tephra occurred in two of the four sites, suggesting that tephra deposition did not create conditions that precipitated a fire event in a consistent way. This research reinforces the notion that disturbance events may have cumulative effects on forest vegetation, but that the impacts of disturbance events need to be felt by similar constituents of the forest ecosystem to be truly additive.

Isotopic analysis of wetland development in the American Southwest.

The analysis of stable isotope and elemental fractions of organic material collected from San Bernardino Ciénega was used to understand the history of vegetation composition and climate change within this desert wetland. A 4000-yr record of sediment buildup, based on four 14C measurements, provides unique opportunities for the study of environmental conditions within an arid landscape and documents climate shifts from drier to wetter conditions in the late Holocene. δ13C, δ15N, and C:N values were measured from a 3.8 m deep sedimentary section to understand the dynamics of vegetation and hydrology in desert wetlands. Through this section we observe δ 13C and C:N values indicating a shift in the dominant source of organic matter within the section: prior to 850 cal. yr BP (below 60 cm), aquatic vascular plants and occasionally terrestrial vegetation were the primary organic sources, whereas freshwater algae were the dominant organic matter source above this level. These values indicate that while conditions remained arid at this locality, the amount of standing water on the ciénega has increased over time. These results document both climate change and vegetation evolution on the ecotone of the Sonoran and Chihuahuan deserts and demonstrate how the study of local sediment accumulation in ciénegas can provide critical information on changing conditions within arid environments.

Response of arboreal pollen abundance to late-Holocene drought events in the Upper Midwest, USA

The impact of short-term climate anomalies and disturbances on past plant communities can be understood using high temporal or contiguous sedimentary pollen analysis. The response of pollen assemblages to decadal- to multidecadal-scale moisture variability was analyzed for the time intervals 3000–2200 and 1400–500 cal. yr BP. The hydroclimate and vegetation history for Minden Bog in southeastern Michigan were reconstructed using analyses of sedimentary testate amoebae and pollen. Results indicated that moisture anomalies accounted for 3% to 24% of the variation in arboreal pollen abundance for Fagus, Pinus, Quercus, and Tsuga. Betula pollen percentages did not appear to be affected by moisture variability. Given the longevity of these taxa, rapid arboreal pollen assemblage responses to moisture variability at decadal to multidecadal timescales were likely due to climate-induced changes in pollen productivity. Our data suggest that pollen–climate relationships may be counterintuitive at these fine temporal scales. For example, Tsuga pollen percentages generally increased during short-term dry events, in contrast to expected decreases in abundance with drying at millennial timescales based on empirical pollen–climate relationships. Results suggest caution should be used when inferring subdecadal to multidecadal climate variation from highly resolved pollen records. Rather, high-resolution pollen data may more accurately represent superimposed plant responses that are the composite of reproductive output nested within long-term plant community compositional changes in response to climatic variation.

High dissimilarity within a multiyear annual record of pollen assemblages from a North American tallgrass prairie.

Abstract Grassland vegetation varies in composition across North America and has been historically influenced by multiple biotic and abiotic drivers, including fire, herbivory, and topography. Yet, the amount of temporal and spatial variability exhibited among grassland pollen assemblages, and the influence of these biotic and abiotic drivers on pollen assemblage composition and diversity has been relatively understudied. Here, we examine 4 years of modern pollen assemblages collected from a series of 28 traps at the Konza Prairie Long‐Term Ecological Research Area in the Flint Hills of Kansas, with the aim of evaluating the influence of these drivers, as well as quantifying the amount of spatial and temporal variability in the pollen signatures of the tallgrass prairie biome. We include all terrestrial pollen taxa in our analyses while calculating four summative metrics of pollen diversity and composition – beta‐diversity, Shannon index, nonarboreal pollen percentage, and Ambrosia:Artemisia – and find different roles of fire, herbivory, and topography variables in relation to these pollen metrics. In addition, we find significant annual differences in the means of three of these metrics, particularly the year 2013 which experienced high precipitation relative to the other 3 years of data. To quantify spatial and temporal dissimilarity among the samples over the 4‐year study, we calculate pairwise squared‐chord distances (SCD). The SCD values indicate higher compositional dissimilarity across the traps (0.38 mean) among all years than within a single trap from year to year (0.31 mean), suggesting that grassland vegetation can have different pollen signatures across finely sampled space and time, and emphasizing the need for additional long‐term annual monitoring of grassland pollen.